1. Field of the Invention
The present invention relates to an endoscope system that obtains information about blood such as an oxygen saturation level of blood, a processor device of the endoscope system, and a method for controlling the endoscope system.
2. Description Related to the Prior Art
In a medical field, an endoscope system is used often for diagnosis and treatment. The endoscope system is constituted of an electronic endoscope to be inserted into a human body cavity, a light source device for supplying illumination light to the electronic endoscope, and a processor device that processes an image signal produced by the electronic endoscope and displays a processed image on a monitor. There is known an endoscope system that carries out not only normal observation for imaging an internal body portion under irradiation with white light (normal light), but also special observation for imaging the internal body portion under irradiation with specific narrow band light (special light).
As the special observation, an oxygen saturation level obtaining technique is known using first light being narrow band light having a wavelength range in which an absorption coefficient is different between oxyhemoglobin and deoxyhemoglobin, and second light having a wavelength range different from that of the first light. The first and second light is applied sequentially to the internal body portion in order to determine an oxygen saturation level of blood. The determined oxygen saturation level is reflected in a normal image obtained in the normal observation, to produce a special image in which a hypoxic region being a cancer-suspected lesion is colored artificially.
The normal image mainly aims to look at details of mucosal structure, including blood vessels, projections and depressions, and the like. Thus, the normal image requires high resolution. In the special image, on the other hand, the hypoxic region extends a certain size, so the high resolution is not required. However, the special image requires high sensitivity to reflected light, because the oxygen saturation level is calculated from the light amount of the first and second light reflected from the internal body portion. For this reason, high-intensity narrow band light emitted from an LD, LED, or light source rivaling the LD or LED is needed as the first and second light. In the case of not using the high-intensity narrow band light, a high-sensitivity image sensor becomes necessary.
Accordingly, US Patent Application Publication No. 2010/0245551 discloses an endoscope system that is provided with two image sensors i.e. a normal image sensor having normal sensitivity and a high-sensitivity image sensor having high sensitivity. In this system, the normal image sensor is used in the normal observation requiring high resolution, while the high-sensitivity image sensor is used in the special observation requiring high sensitivity.
In the US Patent Application Publication No. 2010/0245551, the high-sensitivity image sensor is used for detecting weak autofluorescence distributed through a wide wavelength range. On the other hand, the first and second light used for calculation of the oxygen saturation level has the specific wavelength ranges, so the light amount of the first and second light is not as low as that of the autofluorescence, though is lower than that of the white light. Therefore, in a short-distance view in which a distal end portion of the electronic endoscope is near the body portion, even the normal sensor can detect an enough light amount to calculate the oxygen saturation level. When the normal sensor is adequate, it is desirable to use the normal image sensor having high resolution, instead of the high-sensitivity image sensor.